Re: [for-4.9] Re: HVM guest performance regression

[Juergen Gross <jgross@suse.com>]

On 06/06/17 21:08, Stefano Stabellini wrote:
> On Tue, 6 Jun 2017, Juergen Gross wrote:
>> On 06/06/17 18:39, Stefano Stabellini wrote:
>>> On Tue, 6 Jun 2017, Juergen Gross wrote:
>>>> On 26/05/17 21:01, Stefano Stabellini wrote:
>>>>> On Fri, 26 May 2017, Juergen Gross wrote:
>>>>>> On 26/05/17 18:19, Ian Jackson wrote:
>>>>>>> Juergen Gross writes ("HVM guest performance regression"):
>>>>>>>> Looking for the reason of a performance regression of HVM guests under
>>>>>>>> Xen 4.7 against 4.5 I found the reason to be commit
>>>>>>>> c26f92b8fce3c9df17f7ef035b54d97cbe931c7a ("libxl: remove freemem_slack")
>>>>>>>> in Xen 4.6.
>>>>>>>>
>>>>>>>> The problem occurred when dom0 had to be ballooned down when starting
>>>>>>>> the guest. The performance of some micro benchmarks dropped by about
>>>>>>>> a factor of 2 with above commit.
>>>>>>>>
>>>>>>>> Interesting point is that the performance of the guest will depend on
>>>>>>>> the amount of free memory being available at guest creation time.
>>>>>>>> When there was barely enough memory available for starting the guest
>>>>>>>> the performance will remain low even if memory is being freed later.
>>>>>>>>
>>>>>>>> I'd like to suggest we either revert the commit or have some other
>>>>>>>> mechanism to try to have some reserve free memory when starting a
>>>>>>>> domain.
>>>>>>>
>>>>>>> Oh, dear.  The memory accounting swamp again.  Clearly we are not
>>>>>>> going to drain that swamp now, but I don't like regressions.
>>>>>>>
>>>>>>> I am not opposed to reverting that commit.  I was a bit iffy about it
>>>>>>> at the time; and according to the removal commit message, it was
>>>>>>> basically removed because it was a piece of cargo cult for which we
>>>>>>> had no justification in any of our records.
>>>>>>>
>>>>>>> Indeed I think fixing this is a candidate for 4.9.
>>>>>>>
>>>>>>> Do you know the mechanism by which the freemem slack helps ?  I think
>>>>>>> that would be a prerequisite for reverting this.  That way we can have
>>>>>>> an understanding of why we are doing things, rather than just
>>>>>>> flailing at random...
>>>>>>
>>>>>> I wish I would understand it.
>>>>>>
>>>>>> One candidate would be 2M/1G pages being possible with enough free
>>>>>> memory, but I haven't proofed this yet. I can have a try by disabling
>>>>>> big pages in the hypervisor.
>>>>>
>>>>> Right, if I had to bet, I would put my money on superpages shattering
>>>>> being the cause of the problem.
>>>>
>>>> Seems you would have lost your money...
>>>>
>>>> Meanwhile I've found a way to get the "good" performance in the micro
>>>> benchmark. Unfortunately this requires to switch off the pv interfaces
>>>> in the HVM guest via "xen_nopv" kernel boot parameter.
>>>>
>>>> I have verified that pv spinlocks are not to blame (via "xen_nopvspin"
>>>> kernel boot parameter). Switching to clocksource TSC in the running
>>>> system doesn't help either.
>>>
>>> What about xen_hvm_exit_mmap (an optimization for shadow pagetables) and
>>> xen_hvm_smp_init (PV IPI)?
>>
>> xen_hvm_exit_mmap isn't active (kernel message telling me so was
>> issued).
>>
>>>> Unfortunately the kernel seems no longer to be functional when I try to
>>>> tweak it not to use the PVHVM enhancements.
>>>
>>> I guess you are not talking about regular PV drivers like netfront and
>>> blkfront, right?
>>
>> The plan was to be able to use PV drivers without having to use PV
>> callbacks and PV timers. This isn't possible right now.
> 
> I think the code to handle that scenario was gradually removed over time
> to simplify the code base.

Hmm, too bad.

>>>> I'm wondering now whether
>>>> there have ever been any benchmarks to proof PVHVM really being faster
>>>> than non-PVHVM? My findings seem to suggest there might be a huge
>>>> performance gap with PVHVM. OTOH this might depend on hardware and other
>>>> factors.
>>>>
>>>> Stefano, didn't you do the PVHVM stuff back in 2010? Do you have any
>>>> data from then regarding performance figures?
>>>
>>> Yes, I still have these slides:
>>>
>>> https://www.slideshare.net/xen_com_mgr/linux-pv-on-hvm
>>
>> Thanks. So you measured the overall package, not the single items like
>> callbacks, timers, time source? I'm asking because I start to believe
>> there are some of those slower than their non-PV variants.
> 
> There isn't much left in terms of individual optimizations: you already
> tried switching clocksource and removing pv spinlocks. xen_hvm_exit_mmap
> is not used. Only the following are left (you might want to double check
> I haven't missed anything):
> 
> 1) PV IPI

Its a 1 vcpu guest.

> 2) PV suspend/resume
> 3) vector callback
> 4) interrupt remapping
> 
> 2) is not on the hot path.
> I did individual measurements of 3) at some points and it was a clear win.

That might depend on the hardware. Could it be newer processors are
faster here?

> Slide 14 shows the individual measurements of 4)

I don't think this is affecting my benchmark. It is just munmap after
all.

> 
> Only 1) is left to check as far as I can tell.

No IPIs should be involved.


Juergen

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